A fundamental difficulty in operating any common type of internal combustion engine on multiple fuels is that engine design, operating cycle, and fuel combustion properties are tied together. Highly efficient engines take advantage of inherent fuel properties by the nature of their design. Hydrocarbon blends used for fuel are categorized by performance specifications based on physical measurements of the fuel, and not on the chemical composition of the fuel. As such, the blend for any given fuel will vary so long as the relevant performance specifications for such fuel remain essentially constant.
Internal combustion engines generally comprise three common types. One type of internal combustion engine comprises a spark ignition cycle (Otto cycle) while a second type comprises a compression ignition cycle (Diesel cycle) and a third type comprises a gas compression-ignition cycle, such as a gas turbine engine, a pulse jet engine, a ramjet engine, and a scram jet (supersonic combustion ramjet) engine.
Some fuels, such as gasoline, for spark ignition engines, are commonly characterized by the performance specification referred to as octane number which indicates the fuel's resistance to early ignition and knock of the fuel/air mixture in the engine. High values of octane number for a fuel for spark ignition engines are required to avoid early ignition of the fuel/air mixture in the engine which leads to the problem commonly known as combustion knock in the engine. On the other hand, fuels for compression ignition engines, commonly known as diesel fuel, are generally characterized by the performance specification of cetane number, which indicates the ease of ignition of the fuel/air mixture in the engine. A high cetane number is desired for the fuel of a compression ignition engine to avoid delayed ignition of the fuel/air mixture which leads to incomplete combustion of the fuel/air mixture commonly perceived as smoke production by the engine. Other fuels, such as those for gas turbines, are characterized by the performance specification JP-5, JP-8, JP-10, etc., which are specifically suited for gas turbine engines.
An octane rating number is a measure of the auto-ignition resistance of gasoline and other fuels used in spark ignition internal combustion engines. It is a measure of anti-detonation of a gasoline or fuel. Octane number is the number which gives the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, which would have the same anti-knocking capacity as the fuel which is under consideration. An octane rating number for a fuel for a spark ignition internal combustion engine is a performance specification, used specifically to categorize blends of gasolines, that is based on physical measurements made on running a test engine on a specific blend of fuel (gasoline). The most common type of octane rating is the Research Octane Number (RON) which is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of iso-octane and n-heptane. Another type of octane rating is called Motor Octane Number (MON), or the aviation lean octane rating, which is a measure of how the fuel (gasoline) behaves when under load. The MON octane rating number for a fuel (gasoline) is determined by using a similar test engine to that used to determine the RON octane number for a fuel (gasoline) but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to stress the fuel's knock resistance. Depending on the composition of the fuel, the MON octane rating of current fuels (gasolines) will be about 8 to 10 points lower than the RON octane rating for the same fuel.
In most countries (including Europe and Australia) the octane rating number for a fuel for a spark ignition internal combustion engine is the RON octane rating number for the fuel, but in the United States, Canada and some other countries the octane rating number is the average of the RON octane rating and the MON octane rating for the fuel (gasoline), sometimes referred to as the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON, or (R+M)/2). because of the 8 to 10 point difference of the octane rating for fuel (gasoline) using the MON octane rating, fuel in the United States will have an octane rating about 4 to 5 points lower than the octane rating of the same fuel elsewhere. Typically, spark ignition internal combustion engines are designed to operate on fuel (gasoline) blends having a regulated octane number (average of motor and research) in the range of 87 to 93. Various combinations of hydrocarbons can have the same octane number if they perform the same in an engine. Spark ignition internal combustion engines require a fuel that is not easily ignited by the high gas temperatures created during the compression stroke or during the process of combustion, allowing for a controlled combustion along a “flame front” in the cylinder and combustion chamber of the engine. High compression ratio spark ignition internal combustion engines require high octane number fuel, based on any octane rating method to deter mine the octane rating for the fuel, making the fuel knock resistant (e.g., having delayed combustion characteristics) for the engine to achieve a higher efficiency and deliver more power.
Cetane number, or CN, is a measurement of the combustion quality of fuel (diesel) used for compression ignition engines. The cetane number of a fuel is defined as the percentage by volume of normal cetane in a mixture of normal cetane and alpha-methyl naphthalene which has the same ignition characteristics (ignition delay) as the test fuel when combustion is carried out in a standard compression ignition engine under specified operating conditions. Cetane number is a measure of a fuel's ignition delay (the time period between the start of injection and the start of combustion (ignition) of the fuel). Cetane numbers are only used for relatively light distillate diesel oils.
Generally, compression ignition (diesel) engines run well using fuels having a rating with a cetane number (CN) from 40 to 55. Since fuels having a higher cetane number which have shorter ignition delays providing more time for the fuel combustion process to be completed, higher speed compression ignition (diesel) engines operate more effectively with higher cetane number (CN) fuels. There is no performance or emission advantage when the CN of a fuel is raised past approximately 55 as the fuel performance plateaus. In the United States currently, fuels for compression ignition (diesel) engines are available having a cetane number (CN) in the range of 44 to 50 while in Europe, the fuels for compression ignition (diesel) engines must have a cetane number (CN) rating at a minimum of 51.
Again, various combinations of hydrocarbons can have the same cetane number if they perform the same in an engine. Normal operation in a typical compression ignition (diesel) engine requires a fuel with regulated cetane number in the range of 40 to 60 with cetane numbers up to 50 being referred to as premium. Since a high cetane number fuel is easy and quick to ignite with high gas temperatures, compression ignition engines are designed to ignite the fuel, after it is injected, with the high gas temperatures in the cylinder and combustion chamber of the engine created during the compression stroke of the engine igniting the injected fuel. As the lower limit for fuel for compression ignition engines is a cetane number (CN) of 40, the use of lower cetane number fuels (less than 40) results in increased exhaust smoke production, difficulty in engine starting, especially in cold weather or high-altitudes, rough engine operation due to large cycle-to-cycle variations in power output, accelerated engine deposits from incomplete fuel combustion, reduced engine efficiency due to incomplete fuel combustion, and accelerated lube oil sludge formation form incomplete fuel combustion. Consequently, most compression ignition engine manufacturers recommend the use of diesel fuels with a cetane number of at least 40.
It can be seen that a high-octane number fuel inherently has a low cetane number (CN) and vice versa. As a result, fuels intended for operation in compression ignition engines are poorly suited for use in spark ignition engines, and fuels intended for use in spark ignition engines are poorly suited for use in compression ignition engines.
In yet another category, fuels for gas turbine engines, commonly referred to as jet fuels such as JP-5, JP-8 and JP-10 are also categorized using performance specifications, although different ones are specifically suited for their use in gas turbine engines. Neither cetane number nor octane number is used in the specification of fuels for gas turbine engines. As such, specifications for fuels for gas turbine engines are not regulated and held constant, as each batch of refined fuel varies as to chemical composition. The chemical composition of any batch of fuel can vary as long as the appropriate performance specifications for gas turbine operation are met. Gas turbine fuels exhibit variable octane numbers and cetane numbers, typically having low octane numbers and cetane numbers lower than 40. As a result, the use of fuels for gas turbine engines in any internal combustion engines of either compression or spark ignition type is problematic.